Vol.36,No.1(2014)17‐21
TribologyinIndustry
www.tribology.fink.rs
RESEARCH
NormalForceInfluenceon3DTexture
ParametersCharacterizingtheFriction
CoupleSteel–PBT+10%PTFE
C.Georgescu a,L.Deleanu a,C.Pirvu a
a DunareadeJos”UniversityofGalati,Romania.
Keywords:
PBT+PTFEmaterial
Surfacetextureparameters
Block‐on‐ringtest
Drysliding
Correspondingauthor:
LorenaDeleanu
DunareadeJos”UniversityofGalati,
Romania
E‐mail:[email protected]
ABSTRACT
Thisstudypresentstheinfluenceofthenormalforceonthesurfacequality
of the friction couple steel – polybutylene terephthalate (PBT) + 10 %
polytetrafluoroethylene(PTFE).Therewerecalculatedtheaveragevalues
oftheamplitudeandfunctionalparameters,asobtainedfrominvestigating
squareareasontheweartracks,withthehelpofaproposedmethodology,
forinitialandtestedsurfacesgeneratedontheblocksandoncounterpart
ring made of rolling bearing steel, for the following test conditions: three
normalforces(F=1N,F=2.5NandF=5N),threeslidingspeeds(v=0.25
m/s,v=0.50m/sandv=0.75m/s)andaslidingdistanceofL=7500m.
Theconclusionoftheresearchstudywasthatthetestednormalforcerange
hasaninsignificantinfluenceonthesurfacequalityforthetestedmaterials
andparameters. This friction couple could be recommended for variable
conditions(speedandload)indryregimes.
©2014PublishedbyFacultyofEngineering
1. INTRODUCTION
Inactualapplicationforbearingsandsealsindry
regime, a self‐lubricating polymeric material
slides on a hard surface, proved to be
tribologicallyefficientascomparedtothesliding
of a polymeric material on itself [1,2]. The
adhesionandabrasioncomponentsofthefriction
and wear processes sinergically influence
themselves. Forinstance,extendofthejunctions
depends on the elasto‐plastic deformation of the
asperities[3‐5]andtheycouldnotbeseparated.
For the polymer‐metal contact, the deterioration
of the polymer by elasto‐plastic deformation is
more intense and the adhesion component
increases for the harder surfaces [6]. The
generated transfer film characteristic for the
polymer‐metal friction couple, also changes the
surfacetexture,dependingofthepolymernature
andtheworkingconditions[1,2,7].
There are many published studies on the
tribological behavior of polymeric materials [7‐
9],butfewofthemdealwiththeinfluenceofthe
surface
texture
on
the
tribological
characteristics of the polymeric materials and
even fewer reported how the working
conditions affect the surface texture. In 1970,
Pooley and Tabor (quoted in [1]) pointed out
thatforPTFE,thevalueofthefrictioncoefficient
17
G.Georgescuetal.,TribologyinIndustryVol.36,No.1(2014)17‐21
is only slightly affected by the surface quality
wheninvolvingrelativelysmoothones,butwith
rough surfaces the wear and the friction are
intensified. Till now, the terms "smooth" and
"rough" were used only in a qualitative way and
therearenorecommendedvaluesofthetexture
parametersforparticularapplications.
Experimentalstudiesprovedthatachangeofthe
textureparameterscouldsignificantlyaffectthe
friction and the wear. For instance, Chowdhury
et al. [10] concluded that the values of friction
coefficient and wear rate are different for
smooth and rough counterface pins and type of
materials,thereforeanappropriatelevelofload,
sliding speed and an appropriate counterface
texture of the selected materials, friction and
wear may be kept to a lower value to improve
the contact durability. Even static coefficient of
friction is influenced by the particularities of
surface texture as resulted from the
manufacturingprocess[13].
There is why the authors of this research
considerthetextureevaluation,beforeandafter
testing, necessary for understanding and
directing the tribological processes. Many
polymeric friction couples are working with
frequent starts and stops and the evolution of
the surface texture is of great importance for
improving the reliability and the durability of
thesetribosystems.
Forpolymericfrictioncouplesandespeciallyfor
polymer – metal contacts, the wear could be
related both to the amplitude and functional
parameters.
As resulted from the studied documentation
[14,15], the surface quality is frequently
described by parameters as Sa (arithmetic
average of absolute values) and Sq (root mean
squared). The authors’ estimates that for
studying the worn surfaces and for obtaining
correlations among the surface parameters and
thetestingconditions,thefollowingparameters
aremoresuitable:theparametersrelatedtothe
maximum values of the topography (Sz – the
height difference between the highest and
lowest heights in the investigated area, Sv – the
largest pit height, Sp – the largest peak height)
and the functional parameters (Svk – reduced
valley depth, Sk – core roughness depth, Spk –
reducedsummitheight).
18
2. MATERIALANDTESTINGMETHODOLOGY
The friction and wear behaviour of PBT sliding
against steel was evaluated with the help of a
UniversalMicro‐TribometerUMT‐2andablock‐
on‐ring tribotester. The geometry of the
frictionalcoupleisgiveninFig.1.
Load
sample
rotating ring
Fig. 1. The shapes and dimensions of the friction
coupleblock‐on‐ring.
The polymeric blocks are prisms of 16.5 mm ×
10 mm × 4 mm and they were obtained by
injectionatICEFSSavinesti,Romania,according
to the specifications of the producer from
traction samples, cutting the blocks from the
middleparallelzoneofthem.
The polymeric blend has 90 % (wt) PBT, the
commercialnamebeingCrastin6130NC010(as
supplied in grains by DuPont) and 10 % (wt)
PTFE, commercial grade NFF FT‐1‐1T®
Flontech,havingtheaveragesizeoftheparticles
~20μm.
Theotherelementofthefrictioncouplewasthe
external ring of the tapered rolling bearing KBS
30202 (DIN ISO 355/720), having the
dimensionsofØ35mm×10mmandtheywere
madeofsteelgradeDIN100Cr6,having60‐62
HRCandRa=0.8μmontheexteriorsurface.
There were selected the following test
parameters:threeslidingspeeds(v=0.25m/s,v
=0.50m/s,v=0.75m/s),threeappliedloads(F
=1.0N,F=2.5N,F=5.0N),theslidingdistance
being L = 7500 m for each test done at room
temperatureandinalaboratoryenvironment.
G.Georgescuetal.,TribologyinIndustryVol.36,No.1(2014)17‐21
Inordertodothisstudy,theprofilometerLaser
NANOFOCUSμSCAN[16]wasused.
For parameters' calculation it was used the
software SPIP 5.1.11 [17]. Fig. 2 presents a
virtual (rebuilt) image of the investigated zone
withthehelpofthissoftware.
3. EXPERIMENTALRESULTS
Taking into account that PTFE has lower
mechanicalpropertiesascomparedtoPBT[11],it
wasconsiderednecessarytostudytheinfluenceof
the normal force on the surface quality of this
polymericblend,beforeandaftertesting.
14
12
10
m 8
µ
6
4
2
0
PF10
L = 7500 m
Initial surface
Sa
14
12
10
m 8
µ
6
4
2
0
a) Theinitialsurface
14
12
10
m 8
µ
6
4
2
0
b)Thewornsurface(F=5N,v=0.25m/s,L=7500m)
Fig.2.Virtualimagesofthepolymericblocksmadeof
PBT+10%PTFE.
Sq
Sp
F=5.0N
14
12
10
8
6
4
2
0
Sp
Sq
Sv
Sz
Sv
Sz
PF10
L = 7500m
v = 0.75 m/s
F=1.0N
F=2.5N
F=5.0N
Sa
Sz
PF10
L = 7500 m
v = 0.5 m/s
F=2.5N
Sq
Sv
PF10
L = 7500 m
v = 0.25 m/s
F=1.0N
Sa
μm
Measurementsweredoneforblocksmadeofthe
polymeric blend PBT + 10 % PTFE and for the
external rings of tapered rolling bearings, both
elements being involved in block‐on‐ring tests,
forbothnon‐wornandwornsurfaces.
For evaluating the 3D parameters involved in
thisstudy,therewereselectedthreezones,each
of 500 m  500 m for the polymeric blocks
and of 100 m  100 m for the metallic rings,
these being reduced for reason of the surface
curvature.All3Dmeasurementsweredonewith
a step of 5 m. The distance between lines for
3D measurements was also 5 m. The 3D
parameters are calculated for all the values z(x,
y), measured on one area of 500 m  500 m
on the block and one area of 100 m  100 m
onthesteelring.
Sp
F=1.0N
F=2.5N
F=5.0N
Sa
Sq
Sp
Sv
Sz
Fig.3.Theinfluenceofthenormalforceontheaverage
valuesofthedimensionalamplitudeparametersforthe
blocksmadeofPF10(PBT+10%PTFE).
Figures 3, 4 and 5 present the average values of
the amplitude and functional parameters,
obtained with the help of the proposed
methodology, for the initial and tested surfaces
generated on the blocks made of PF10 (material
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G.Georgescuetal.,TribologyinIndustryVol.36,No.1(2014)17‐21
symbol for the polymeric blend PBT + 10 %
PTFE),forthetestedconditions:threeforcesand
threeslidingspeedsandaslidingdistanceofL=
7500m.
The wear track surfaces are characterized by
parametricvalues2...3timeslowerthanthoseof
theinitialsurfacesastheywereobtainedbythe
mouldingtechnology.
Thesurfacequalityofthismaterialisonlyslightly
dependent on the normal force, at least for the
testedvalues(F=1N,F=2.5NandF=5N).
SaandSqhaveveryclosevalues,regardlessthe
force values, but Sp and Sz present a slight
decrease when the force increases, for the test
done with the sliding speeds of v = 0.25 m/s
andv=0.5m/s.
4
6
4
PF10
L = 7500 m
Initial surface
3
m2
µ
2
1
0
Ssk
Sku
-2
0
Spk
4
6
PF10
L = 7500 m
v = 0.25 m/s
3
μm
4
2
Ssk
Sku
-2
4
PF10
L = 7500 m
v = 0.5 m/s
Spk
Sk
Svk
F=1.0N
F=2.5N
F=5.0N
PF10
L = 7500 m
v = 0.5 m/s
1
Sku
-2
6
0
Spk
4
PF10
L = 7500 m
v = 0.75 m/s
3
Sk
Svk
F=1.0N
F=2.5N
F=5.0N
PF10
L = 7500 m
v = 0.75 m/s
m2
µ
2
1
0
Ssk
Sku
Fig. 4. The influence of the normal force on the
average values of the dimensional amplitude
parametersfortheblocksmadeofPF10.
20
F=1.0N
F=2.5N
F=5.0N
PF10
L = 7500 m
v = 0.25 m/s
m2
µ
Ssk
0
3
0
-2
Svk
2
4
2
4
Sk
1
0
6
PF10
Initial surface
0
Spk
Sk
Svk
Fig. 4. The influence of the normal force on the
average values of the 3D functional parameters for
theblocksmadeofPF10.
G.Georgescuetal.,TribologyinIndustryVol.36,No.1(2014)17‐21
Sku (surface Kurtosis) values greater than 3.0
indicate narrower height distribution due to
the particular ductile fracture of the polymer
during adhesion – abrasion wear. Ssk (surface
Skewness) has values oscillating around zero,
indicatingsymmetricheightdistributions.IfSsk
<0,thebearingsurfacehasholesandifSsk>0
itisaflatsurfacewithpeaks.
It was noticed a slight decrease of the
functional parameters when the load
increases,fortestsdonewiththeslidingspeed
of v = 0.25 m/s. For the other two tested
speeds (v = 0.50 m/s and v = 0.75 m/s), this
poor dependence on the normal force was
noticed only for Svk. The greater forces make
this parameter to decrease and this tendency
could be justified by the elasto‐viscous
behaviourofthepolymericblend;itispossible
thatthepassingofthehardasperitieslaterally
moves the softer material of the counterpart
accompanied by an elevation of the valley
bottoms between asperities, process also
reportedin[12,1].
4. CONCLUSION
ForthefrictioncouplepolymericblendPBT+10
%PTFEslidingagainststeel,therewasfoundno
significant influence of the normal force on the
surfacequalityoftheelementmadeofpolymeric
composite, for the testing conditions: range of
force 1 N ... 5 N, range of speed 0.25 m/s ... 0.75
m/sandtheslidingdistance7500m.
The obtained results recommend the tested
materialsforfrictioncouplesfunctioningunder
variable conditions (speed and load) in dry
sliding. Also, this research points out the
importance of correlating a set of texture
parameters for analysingthe surface quality in
tribologicalapplications.
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